Expandable ethylenically unsaturated polymer particles are conventionally prepared by the process of dispersing in water ethylenically unsaturated polymer particles to form a suspension wherein a volatile liquid organic expanding agent having a boiling point lower than the softening point of the polymer particles is then added and impregnated into the particles as disclosed in the specification of U.S. Pat. Nos. 2,893,963 and No. 2,950,261. But there has been a tendency of ethylenically unsaturated polymer particles, during the impregnation, to agglomerate or stick together to form large clumps because of the solvent action of the expanding agent during the impregnation of the above-mentioned polymer particles with the abovementioned expanding agent. The agglomeration is further accelerated when the impregnation temperature is raised to over 80.degree. C in order to reduce the impregnation period. In order to prevent agglomeration of the polymer particles during the impregnation step, a suspension stabilizer is generally used. As to such suspension stabilizers, it is known to use suspension stabilizers which are generally available to make ethylenically unsaturated polymer particles by suspension polymerization of ethylenically unsaturated monomers. Examples of such stabilizers include, fine powders which are slightly water-soluble, such as inorganic compounds of calcium phosphate, magnesium pyrophosphate, calcium carbonate, calcium silicate and bentonite; fatty acid salts such as calcium stearate and zinc stearate, and organic fine powders such as fatty acid bis-amide e.g., ethylene-bis-stearoamide. The above-mentioned suspension stabilizers, however, are not suitable from the economical stand point, since each of the suspension stabilizers have to be used in great quantities when used alone, in order to prevent the agglomeration of the polymer particles caused by expanding agent. Furthermore, agglomeration is further accelerated when the impregnation temperature is increased in order to reduce the impregnation period, and so fine powders have to be used much more than above, which provides no industrial merits. As a result, the increased cost is not desirable and furthermore, the use of large quantities of stabilizer presents problems for waste water treatment.
It has been proposed to use suspension stabilizers together with a surface active agent in order to prevent agglomeration of the polymer particles by using as little suspension stabilizers as possible, as in the case of using a fine powder of inorganic compounds as suspension stabilizers. For example, the specification of U.S. Pat. No. 2983,692 shows that the effect of suspensions is increased by using anionic surface active agents together with fine powders of inorganic compounds. The use of anionic surface active agents gives assistance in dispersing the inorganic compounds and prevents agglomeration of the polymer particles. It is well known to employ a process using calcium phosphate and sodium dodecylbenzenesulfonate.
The process of using ordinary surface active agents is not suitable, although the process is effective to prevent agglomeration of the polymer particles, since vigorous foaming is caused by the surface active agent when recovering and/or releasing the remaining expanding agent after the impregnation step. It is, therefore, difficult to recover and/or release the expanding agent. Therefore, it is necessary to use anti-foam agents in order to break the foam, and to provide expensive equipment in order to completely remove the surface active agent during the treatment of any waste water.
Fine powders of organic compounds, such as calcium stearate and ethylene-bis-amide can also be used together with anionic, nonionic and amphoteric surface active agents, but these compounds are not as effective as mixtures of inorganic compounds and anionic surface active agents.
Water-soluble organic compounds such as partially saponified polyvinylalcohol, polyacrylate, polymethacrylate, polyvinylpyrolidone, carboxymethyl cellulose, ethyl cellulose, methyl cellulose and sodium alginate may also be used as suspension stabilizers without causing agglomeration of the polymer particles. However, the upper temperature limit of such a process using the above compounds is up to 80.degree. C. When the process is carried out above 80.degree. C, impregnation of the expanding agent becomes difficult. Therefore, it is impossible to reduce the period of impregnation. It is not desirable to use the above described water-soluble organic compounds, since the removal of such organic compounds is very difficult.
The specification of Japanese Patent Publication No. 25147/1972 shows a process which prevents agglomeration of polymer particles during the step of impregnating particles with an expanding agent, by means of coating the surfaces of polymer particles with an oily agent such as mineral oil, polyethyleneglycol, dibutyl phthalate, etc., and coating the oily surface with a fine powder (which is insoluble in water and expanding agents) such as calcium carbonate, calcium silicate, magnesium carbonate, silica, talc, etc., and suspending the resulting coated particles in water. This method, however, has disadvantages in that it is necessary to subject the polymer particles to a two step surface coating in advance, and to use water-soluble organic compounds, e.g., polyvinylalcohol.
After the investigation of a process for improving the above-mentioned processes, we have found that excellent suspension stability and reduced period of impregnation are provided by using a small amount of fine powdered calcium carbonate having its surface treated with resin acid and it is not necessary to use a surface active agent and/or water-soluble organic compound having protective colloidal properties.